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[100521665]

I am having trouble to properly simulate metasurfaces with more than 1 centric structure, getting transmission plots like this:

Where the 'global picture' is still adequate, but seems like there is trouble in converging, resulting in those sinusoidal fluctuations. The figure I am trying to simulate is the following (taken from He et al. Physical Review B 98, 2018):

Being present only Si for the cylinders and SiO2 for the substrate (Palik). The parameters of the simulation involve periodic conditions in both x and y axes, while I have chosen a PML for the z axis, with 'steep angle' and 36 layers. I have tried to increment the auto shutoff time and do some other simplifications, but besides reducing the mesh size I have found no help in whatever I was doing. 

 

Any help would be appreciated! 

[Guilin Sun]

This is a common question regarding to "sinusodal flutuation" in transmission/reflection. The common cause is, the simulation time is not long enough, or the autoshutoff min is not small enough. Please significantly increase the simulation time, say 100 times more, and let the autoshutoff min to terminate the simulation. You can use the "check point"  https://optics.ansys.com/hc/en-us/articles/360034382534-FDTD-solver-Simulation-Object  to reduce the autoshutoff min and resume the simulation to improve the result, which is much more efficient.

You have chosen the steep angle PML with 36 layers, which is good. However, such device may have strong scattering at larger angle, so please use more PML layers. It is important when you used finer mesh. Make sure the PML is thick enough, say half wavelength.

Finally, maybe you have already did so, but I still suggest to check if the SiO2 substrate has been extended to outside of PML. Otherwise it will create oscillation due to cavity effect.